In the refining of molten steel in a basic oxygen furnace (BOF) a slag blanket or covering is provided over the molten steel contained therein. The slag performs a multiplicity of functions in the refining process, such as scavenging undesirable elements from the molten steel and maintaining the high temperature of the steel.
When the molten steel is suitably refined and ready for tapping into a metallurgical vessel, such as a ladle, with the slag blanket remaining essentially on the free surface of the steel, the BOF vessel is tilted allowing the molten steel to exit through a taphole in the side of such vessel. A consequence of such tapping process is that a portion of the slag is transferred to the ladle. In fact, a certain amount of slag from the BOF was believed desirable to provide satisfactory insulation to the underlying molten steel in the ladle. However, quality requirements, such as desulfurized and very clean steels demand that only a limited amount of BOF slag enter the ladle. For example, the oxidized slag interferes with desulfurization. Thus, while it is important to control the amount of slag which is transferred to the ladle from the BOF, that which is transferred must eventually be separated from the molten steel.
In a typical practice followed today in the steel industry, the ladle is first filled with molten steel from the BOF until the slag layer above the steel is about 2 to 10" deep. An inert gas, such as argon, is then bubbled through the molten steel to homogenize it followed by a waiting period. Such procedure helps to free entrapped slag from the liquid steel and reestablish, or redefine, the slag-metal interface. The ladle is then drained into a tundish through a refractory pipe or shroud at a predetermined rate depending on the casting rate. The draining rate of the ladle is controlled with a slide gate valve at the bottom of the discharge orifice to maintain a constant steel level in the tundish. As the molten steel is drained from the ladle, the slag layer remains above the steel until the level of molten metal approaches the bottom of the ladle. The slag is then drawn down into the discharge stream and becomes entrapped in the steel. This causes unacceptable, impurities and surface defects in the final product. Consequently, draining of the ladle must be prematurely stopped, whereby the ladle is not completely drained and several tons of molten steel are lost in the process.
Through modeling studies on liquid flow using water and oil to simulate steel and slag, respectively, it has been observed that when liquid is drained from a hole in the bottom of a vessel, a vortex or a dip of the surface can occur above the drain hole. Vortexing, which starts with circulation of liquid in the vessel, can fully develops into the drain hole. The critical height, i.e., the height of the liquid where this fully developed vortex forms, increases with increasing outlet velocity and rotational flow or circulation in the vessel. It has also been observed through modeling studies, with or without the presence of a vortex, that near the end of drain the liquid slag layer collapses into the stream of liquid steel that is entering the drain hole. This mechanism is known as exceeding the limit of selective withdrawal, and the critical height or depth of heavier liquid at which this occurs is a function of the velocity of discharge, diameter of the discharge orifice, and density difference of the fluids.
From studies simulating draining of a ladle for continuous casting, the entrainment of slag is believed to be wholly caused by the collapse of the slag layer into the drain hole, or alternately, at the limit of selective withdrawal. Accordingly, a prime object of this invention is to significantly reduce the yield loss caused by collapse of the slag layer into the drain hole.